1. Technical Field
The present invention relates to a switching power supply device such that an impeding of the reception of an AM radio broadcast due to power supply noise is avoided, and a reduction in power supply noise is achieved.
2. Related Art
It is undeniable that, in a switching power supply device, high frequency power supply noise (electromagnetic interference noise; EMI noise) is generated accompanying a switching operation in a semiconductor switching element. In the event that the frequency of this kind of power supply noise overlaps an AM radio frequency bandwidth, it is a cause of hindering the reception of the AM radio broadcast (AM reception impedance). Therefore, to date, it has been proposed that reception impedance is avoided by demodulating an AM signal from a sideband on a side of an AM radio receiver in which high frequency power supply noise from the switching power supply device is not included (for example, refer to JP-B-63-158929).
Also, it has also been proposed that electromagnetic interference noise (power supply noise) accompanying a switching operation is reduced by applying random fluctuation (jitter) to a switching frequency in a switching power supply device (for example, refer to JP-A-2003-153526).
However, even when reducing power supply noise using the switching frequency jitter control technology introduced in JP-A-2003-153526, impeding of the reception of an AM radio broadcast due to the power supply noise is not necessarily avoided. In fact, when controlling the jitter of the switching frequency, the power spectrum of the power supply noise spreads over the frequency axis, without concentrating at a specific frequency. Because of this, the possibility of the power supply noise frequency overlapping (interfering with) the AM radio frequency bandwidth increases, and there is conversely concern that the power supply noise frequency constitutes a factor in impeding the reception of the AM radio broadcast.